In the field of embedded devices such as consumer electronics devices, a power saving technique is known in which the operating voltage of a processor of an embedded device is dynamically lowered or the operating frequency is lowered when the amount of processing performed by the processor is small. In addition, a technique of shutting off the power to a device when no access is made thereto within a designated period of time is also known.
Furthermore, a method of saving power by shifting to a mode called “sleep” when there is no input (e.g., keyboard input) from a user within a certain period of time is known. In the sleep mode, the operating frequency and the supply voltage of a processor are reduced, and the processor is thereby shifted to a low power consumption mode so that the power consumption can be reduced. With this method, the state of the processor and the information in a volatile memory are stored in the device, and thus the normal mode can be recovered in a short time in response to the user's input.
In a mode called “hibernation,” unlike in the sleep mode, the information of the processor and the memory is stored in a non-volatile storage device (such as a hard disk), and then the power to the processor and the memory is shut off so that a mode of still lower power consumption than the sleep mode can be achieved. If the user presses a power switch during the hibernation mode, the embedded device judges that the user has instructed a shift from the hibernation mode to the normal mode. The embedded device reads the state of the processor and the information in the memory stored in the storage device and writes them back onto the processor and the memory, so that the mode previous to the hibernation mode can be recovered.
As described above, techniques of reducing power consumption when the user is not using the device have been developed. When there is no input from the user within a predetermined period of time, the device is shifted to the sleep mode or the hibernation mode for reducing the power consumption.
Meanwhile, even when the user is performing input, there is a period of time, in units of several milliseconds, in which the processor or the device is not in operation. The speed of typing performed by a human being is one hit at most in several tens of milliseconds. Thus, when the user is typing on the keyboard, the processor may be in a no-task state (i.e., in an idle mode), until the next key is pressed down. If the power to the processor or the memory can be shut off in units of several milliseconds without waiting for the predetermined period of time to elapse unlike in the conventional technologies, the power consumption can be further reduced.